At 5:12 a.m. on April 18, 1906 the rocks snapped six to nine miles below the surface of the earth. The land was torn apart from Shelter Cove in Humboldt County to Hollister in San Benito County. The epicenter was located on the San Andreas Fault at the boundary of San Francisco and San Mateo counties. The magnitude 7.8 earthquake, lasting forty to sixty-five seconds, was not the strongest ever experienced in California, or for that matter, in the United States. But it was the closest to a major population center.
Fortunately, people were at home in bed and not on crowded city streets or in offices and schools.

The seismic waves spread outward. Ship captains thought they had collided with rocks. City streets heaved up and down as if subjected to ocean swells. Then the ground twisted "like a top while it jerked this way and that, and up and down and every way," said a policeman. The damage was immediate and most severe on the "made land," especially marsh and bay lands that had been filled in east of Sansome Street and south of Market Street. The crowded four-story Valencia Street Hotel shrank to one story, trapping most of the occupants.

To the west, the new San Francisco City Hall disintegrated into the surrounding streets. "In the space of a minute a building that was architecturally the largest and most pretentious in the State of California was shaken to the ground almost like a pack of cards," noted a newspaper. "Its walls and pillars collapsed, its copper dome remained standing on its skeleton steel pillars above a chaos of destroyed masonry."
Wrecked city halls were a tradition in San Francisco, as others had been destroyed or damaged in 1865, 1868, and 1989 earthquakes.

Ft. Bragg and Santa Rosa to the north were badly damaged, as was San Jose and Stanford University to the south. At Agnews State Hospital near San Jose, one hundred and twelve patients and staff members were crushed to death. The largest single death toll was caused by faulty construction. "The construction of these [brick] buildings, with their thin walls (in many places devoid of mortar) and light, insufficient wooden framing, indicates a criminal negligence that is appalling," stated a report on the damage at Agnews by the U.S. Geological Survey.